A Malaysian airliner was brought down over eastern Ukraine on Thursday, killing all 295 people aboard and sharply raising stakes in a conflict between Kiev and pro-Moscow rebels in which Russia and the West back opposing sides.

Malaysia Airlines said air traffic controllers lost contact with flight MH-17 at 1415 GMT as it flew over eastern Ukraine towards the Russian border, bound for Asia with 280 passengers and 15 crew aboard. Flight tracking data indicated it was at its cruising altitude of 33,000 feet when it disappeared.

"I was working in the field on my tractor when I heard the sound of a plane and then a bang," one local man at Grabovo told Reuters. "Then I saw the plane hit the ground and break in two. There was thick black smoke."

An emergency worker said at least 100 bodies had been found so far and that debris was spread over 15 km (9 miles). Workers were scouring the area for the black box flight recorders.

"MH-17 is not an incident or catastrophe, it is a terrorist attack," Ukrainian President Petro Poroshenko tweeted. He has stepped up his military campaign against the rebels since a ceasefire late last month failed to produce any negotiations.